(a) Field of the Invention
The present invention relates to a method of producing a copper foil for fine wiring, which has a bonding surface uniformly roughened to a low surface roughness, has good etching properties, enables high density fine wiring and also adheres strongly to base materials.
(b) Description of the Related Art
The bonding surface of copper foil for printed wiring boards is generally roughened by some means previous to lamination onto resin base materials, to increase adhesive strength. Plating methods have been mainly used for roughening electrolytic copper foil. Japanese Patent Application Examined Publication No. 53-39376 (1978) discloses an example of the plating methods. According to the method, first a dendritic copper electrodeposition layer is formed on at least one bonding surface of a copper foil as a cathode by the so-called burning plating in an acidic copper plating bath at a current of the limiting current density or higher, then a smooth copper electrodeposition layer is formed on the dendritic copper electrodeposition layer at a current of a current density lower than the limiting current density (covering plating) to change the dendritic copper into nodular copper and to increase the adhesive strength by the nodular copper. After the formation of the nodular copper, the surface of the copper foil has a larger specific surface than before the electrolysis treatment, and the nodular copper works as an anchor to increase the adhesive strength between resin base materials and the copper foil. When nodular copper is formed on electrolytic copper foil, which generally has a surface (mat surface) rougher than the other surface (shiny surface), current is mainly centered to convexes, and the formation of the nodular copper is centered on the extreme tops of the convexes.
With the recent spread of note type personal computers and pocket telephones, the wiring pattern of printed wiring boards fabricated therein has become increasingly denser and finer to a degree of circuit width and space of 100 xcexcm or less. The use of glass-epoxy printed wiring boards produced by using as resin base materials FR-5 materials having high Tg""s has also been increased. As compared with conventional FR-4 materials, epoxy resins having high Tg""s are more resistive to heat but have lower adhesive strength to copper foil. A means of enhancing the adhesive strength of copper foil to resin base materials is to increase the roughness of the bonding surface of copper foil. However, increasing the surface roughness tends to cause the so-called copper powder-falling off that is the falling of nodular copper even with small abrasion force and the so-called residual copper that is nodular copper left in resin base materials after the etching step in the production of printed circuits.
Japanese Patent Application Examined Publication No. 54-38053 (1979) discloses an improved method of forming a roughened surface by carrying out electrolysis treatment at an approximate limiting current density in an acidic copper plating bath to which a specific amount of at least one metal selected from arsenic, antimony, bismuth, selenium and tellurium is added. Minute projections can be formed by adding a very small amount of arsenic, antimony, bismuth, selenium or tellurium, but are still centered to the convexes on the copper foil. Further, printed wiring boards using the copper foil containing arsenic, antimony, bismuth, selenium or tellurium, which are poisons or deadly poisons, cause environmental pollution on discarding etching waste liquors or the printed wiring boards themselves.
The addition of benzoquinoline to an acidic copper plating bath (Japanese Patent Application Examined Publication No. 56-41196 (1981)) and the addition of molybdenum (Japanese Patent Application Examined Publication No. 62-56677 (1987)) are also proposed, but cannot improve adhesive strength sufficiently.
Japanese patent Application Unexamined Publication No. 8-236930 (1996) discloses a method for solving the problem, wherein electrolysis is carried out at approximate limiting current density in an acidic copper plating bath containing metal ions of at least one metal selected from chromium and tungsten and metal ions of at least one metal selected from vanadium, nickel, iron, cobalt, zinc, germanium and molybdenum, to form a roughening-treated layer containing the metals added. Japanese Patent Application Unexamined Publication No. 11-256389 (1999) discloses a method of carrying out electrolysis at approximate limiting current density in an acidic copper plating bath containing molybdenum ions and metal ions of at least one metal selected from iron, cobalt, nickel and tungsten, to form a layer of burnt deposits (a layer formed by burning plating) containing the metals added.
These methods, however, also cause the phenomena of copper powder-falling off and residual copper, because nodular copper is formed exclusively on the extreme tops of the convexes on raw copper foil.
Etching factor (Ef) is a measure of the etching properties of copper-clad laminates. FIG. 1 is an illustrative sectional view of a circuitized copper foil for explaining etching factor (Ef), wherein a circuitized copper foil A is formed on an electrically insulating base material B by etching a copper foil. When the top width of the circuitized copper foil is WT, the bottom width of the circuitized copper foil is WB and the thickness of the circuitized copper foil is H, Ef=2H/(WBxe2x88x92WT). Circuit patterns having larger Ef have walls that are nearer perpendicular. To make finer wiring, it is preferable to use copper foil having larger Ef.
Ef depends on the thickness of copper foil, the surface roughness of the bonding surface of copper foil or the like. Ef decreases with the increase of the surface roughness of the bonding surface, because the projections on the roughened surface of the copper foil dig into base materials, and necessitate elongated etching time to completely remove them and damage the patterned circuit shape due to the excessively etched circuit walls. If the projections of the roughened surface are not etched off completely, the copper particles remaining in the base materials may cause disconnection or insulation failure in narrowly spaced fine wiring.
Fine wiring, therefore, requires copper foil, which has a uniformly roughened bonding surface of low surface roughness, good etching properties and a high adhesive strength to base materials.
The object of the present invention is to provide a copper foil for fine wiring, which has a uniformly roughened bonding surface of a low surface roughness, does not decrease the adhesive strength between the copper foil and resin base materials, has a high etching factor and can be finely patterned without leaving copper particles in base materials.
Accordingly, the present invention provides a method of producing a copper foil for fine wiring, comprising
treating a copper foil as a cathode by electrolysis in a plating bath (A) containing (1) copper ions, (2) metal ions of at least one metal selected from the group consisting of tungsten and molybdenum, (3) metal ions of at least one metal selected from the group consisting of nickel, cobalt, iron and zinc and (4) 1 to 100 mg/l of chloronium ions, at a current density lower than a limiting current density of the plating bath (A), to form on a bonding surface of the copper foil a composite metal layer comprising (I) copper, (II) at least one metal selected from the group consisting of tungsten and molybdenum and (III) at least one metal selected from the group consisting of nickel, cobalt, iron and zinc; and
forming a roughened layer comprising copper on the composite metal layer by carrying out electrolysis in a plating bath (B) containing copper ions at a current density not lower than a limiting current density of the plating bath (B) to form a dendritic copper electrodeposition layer and then carrying out subsequent electrolysis at a current density lower than the limiting current density of the plating bath (B) to form nodular copper.
Herein, the terms xe2x80x9cthe limiting current density of the plating bathxe2x80x9d mean a current density at which the generation of hydrogen accompanies the cathode reaction for the deposition of the metals or metal compounds.